Electronic structure and magnetic properties of hexaferrites: An <i>ab</i> <i>initio</i> study

Hexaferrites have been of huge research interest because of their technological applications in permanent magnets, data storage in computer hard disks and tapes, microwave devices, and wireless communications. Finding suitable hexaferrite that can exhibit a strong uniaxial magneto-crystalline anisotropy energy (MAE) and high coercivity is still an open question today for improvement of the device efficiency. We study the electronic structure, magnetic properties, and magneto-structural transition in Sr/La hexaferrites using advanced density functional methods. Our calculations suggest that La-substituted compounds enhance the MAE while transforming from P6$_3$/mmc structure (high temperature phase) to P$\bar{6}m$_2$ structure (low temperature phase) compared to {\it sp}-electron based hexaferrites. The underlying physics in the enhancement of MAE relies in the electronic changes resulting from local crystalline environment of Fe-atoms nearest to the La. Using an anisotropic energy density model, we show that the contribution of higher order anisotropy constants plays significant role in describing the experimentally measured MAE.